Speaker
Description
The PIONEER experiment [1] will investigate rate pion decays and aims to measure the branch-
ing ratio $R_{e/μ} ≡ Γ(π^+ → e^+ν(γ))/Γ(π^+ → μ^+ν(γ))$ with a precision of 0.01 % in its first phase.
This marks an improvement by an order of magnitude with respect to the current experimental
uncertainty and would match the uncertainties of the theoretical predictions. This allows to test
lepton flavour universality at an unprecedented level and probe mass scales up to the PeV range.
In the second phase of the experiment, a 0.06% measurement of the branching ratio of the pion
beta decay, $R_{πβ} ≡ Γ(π^+ → π0e^+ν(γ))/Γ($all$)$, will provide another window into potential physics
beyond the SM by exploring the Cabibbo angle anomaly and by providing a theoretically pristine
measurement of |Vud|. In addition, further rare decays involving various exotic particles such as
ALPs or sterile neutrinos can be searched for with unprecedented sensitivities.
The experimental design incorporates lessons learned from the previous generation PIENU [2]
and PEN/PIBETA [3, 4] experiments at TRIUMF and PSI. It involves stopping an intense pion
beam in a novel active target (ATAR), which is surrounded by a 3π sr, 25 radiation length (X0)
electromagnetic calorimeter to determine the energy of the final state products. A tracker is placed
between ATAR and calorimeter to link pion stopping events in ATAR to calorimeter showers. Each
of the subsystems is being actively modeled in simulation to ensure we will be able to reach our
experimental goal.
Here, we present the theoretical motivation for PIONEER, discuss the experiment design, and
show recent results from simulations and a first beam development campaign at the PSI PiE5
beamline.
[1] PIONEER Collaboration, arXiv:2203.01981 [hep-ex]
[2] A. Aguilar-Arevalo et al. (PIENU), Phys. Rev. Lett. 115, 071801 (2015), arXiv:1506.05845 [hep-
ex].
[3] C. J. Glaser et al. (PEN) arXiv:1812.00782 [hep-ex]
[4] D. Počanić et al. Phys. Rev. Lett. 93, 181803 (2004)
